The present invention is primarily directed toward the production of structural components of the type used in the automotive field and it will be described with particular reference thereto; however, the invention is much broader and may be used for forming various structural components from tubular sheet metal blanks. In the past, such structural components were normally produced by stamping, forming and welding. In an effort to obtain complex shapes, such components have been formed by a hydroforming process wherein tubular blanks are provided from sheet steel material having specific initial strength and elongation. The tubular blank is cut to length and pre-bent or preformed into a shape approximating the shape of the finished structural component. The preformed tubular element is loaded into a two piece die closed in a hydraulic press typically having a closing pressure between about 3500-8000 tons. The exposed ends of the tubular blank are sealed and the tube is filled with a water and oil mixture. The internal pressure of the water and oil mixture is raised to a high level in the general neighborhood of 20,000-80,000 psi which pressurized liquid expands the tubular blank into the shape of a steel die cavity machine in two die members of a die set carried by the hydraulic press. The cavities of the two die members have the desired final shape for the structural component so that as the tubular blank is expanded into the cavity, the outer shape of the component captures the shape of the cavity. This process produces a relatively accurate complex outer shape for the structural component. To relieve the fluid pressure, holes are pierced into the formed structural component. Thereafter, the two die members are opened by the hydraulic press and the liquid is drained from the formed structural component. Secondary machinery operations, such as trimming and cutting mounting holes, is then performed to produce a desired component for final assembly. This process is gaining popularity because it forms the component from the inside so complex shapes are possible; however, the total cycle time for hydroforming is at least about 25-45 seconds. The equipment to direct high pressure liquid into the tubular blank is extremely large and expensive. In addition, the die members are expensive machined parts and have a short life. Hydroforming operations have a general limitation that they are used primarily for bending of the tubular blank, since the steel being formed is processed at ambient temperature which limits the maximum strain rate for the metal being formed. Pressure of the liquid used in the hydroforming must be extremely high to deform the relatively cold sheet metal of the tubular blank into simple configurations. Consequently, hydroforming is used primarily for bending and straightening tubular elements into the desired final shape. Even though there are process limitations in using hydroforming to make tubular structural components, a substantial technology field has developed around this process. In a feature of hydroforming, the sheet steel tubular blank is formed into desired shapes while additional material is forced axially into the die cavity so the wall thickness does not drastically decrease as the volume of a given cross section increases during the processing by high pressure liquid.
Hydroforming is the primary prior art constituting the background of the present invention. However, blow forming of plastic sheets has been used for years to produce high volume plastic containers using conventional steel die members. Of course, such die members used in plastic blow forming can not be used for forming steel. For that reason, hydroforming is used for metal, instead of blow forming as used in the plastics industry. The highly developed technology of hydroforming of steel tubes and blow forming of plastic sheets are background of the present invention, but are not economically usable for forming sheet steel tubular blanks into tubular structural components. In addition, these prior processes do not have the capability of controlling the metallurgical characteristics along the length of the tubular blank, as obtainable by the present invention.
Even though hydroforming of sheet steel and blow forming of plastic sheets are the basic background to the present invention, it has been found that certain features of the technology disclosed by Boeing Company for superplastic forming sheet metal plates by high pressure gas are used in practicing the invention. The Boeing Company process is not background information from the standpoint that it is not capable of forming a tubular metal blank into a structural tubular component and is not capable of controlling the metallurgical characteristics of the metal forming the structural tubular component. These are all advantages of the present invention.